What Is Differential Settlement?

Differential settlement is a type of foundation settlement, which occurs when the soil underneath a building moves or shifts. Unlike uniform or “normal” settling, which basically means that the foundation settles at the same rate over its entire footprint area, differential settlement involves a non-uniform movement of the underlying soil, which causes the foundation along with the structure above to settle in an uneven fashion. The most dangerous aspect of differential settlement is that, when left unaddressed, it can lead to serious structural damage and expensive repairs.

How Does Differential Settlement Occur?

As the expansive clay soil around and underneath a home is in constant movement, voids of various sizes can form under different parts of the foundation. Because those voids will weaken the soil, meaning that the ground can no longer provide adequate support for the structure above, they could cause a foundation to shift. Since this movement will eventually push parts of the ground underneath the foundation to other areas, differential settlement often gets worse over time.

Depending on where the voids form under a foundation, they could lead to all sorts of problems. For instance, if a void forms under a structural wall, the wall may start to develop cracks as it settles into that void. If a void develops under one or more piers that support a foundation, the corresponding part of the floor on top will start to sink.

On the other hand, when a large void forms under a perimetral element of a foundation, one corner or side of the foundation could sink. This will lead to sloping or uneven floors. In other cases, a void could develop right under a structural column. As structural columns are subject to different compressive and tensile stresses, a column that lacks adequate support underneath might start to sink gradually. When that happens, all the other structural and non-structural elements that are connected to it can move and shift out of their positions. This could cause extensive structural damage throughout your Houston home.

Assessing the Main Causes of Differential Settlement

Typically, differential settlement is a direct result of soil consolidation. Soil consolidation occurs when the numbers and sizes of the voids or spaces between soil particles are reduced as the soil gets compressed. In general, clay soil compresses under the weight of a home or when it becomes overly wet, with a muddy consistency. In addition, too much water will cause clay soil to expand considerably, which could lead to heaving.

When clay soil dries out, even more consolidation can occur, regardless of whether there are any structural loads on top or not. That’s because this type of soil contains specific minerals that shrink when they dry. But when clay soil dries completely, it also cracks, which means that a series of gaps could form around and under the foundation.

Either of the situations presented above can result in differential settlement. A few factors that could create the ideal conditions for differential settlement to occur include:

• Poor drainage – Due to poor drainage, water can accumulate around and under your home or flow too close to it. Water that accumulates or drains along your foundation can erode the soil, leaving your foundation without the support it needs in order to maintain its level position. As a result, your foundation will begin to settle unevenly. In addition to poor drainage, a leaky pipe or sewer line could also cause soil erosion and differential settlement when left unaddressed.
• Poor soil compaction during the construction process – Typically used to strengthen and support a home’s foundation while promoting better drainage, backfill soil should be very well compacted through a suitable method. When proper soil compaction isn’t possible, different soil stabilization methods can be used to ensure a solid and stable substrate for the foundation.
• Failure to water the foundation properly during a drought – As explained above, any dramatic changes in the moisture content of the soil can lead to differential settlement. By watering your foundation during drought conditions and ensuring that your yard is draining properly during the wet season, you’ll be able to maintain a relatively constant moisture content in the soil around your house throughout the year—and prevent differential settlement.
• Changes in the water table – When the water table rises, clay soil may become softer and more compressible. Conversely, lowering the water table could cause the soil to dry out too quickly. Since soil consolidation can occur in both these cases, significant fluctuations in the water table can induce differential settlement.

How Can You Know if Your Houston Home Has Experienced Differential Settlement?

Diagonal or stair-step cracks in walls and/or foundation, sticking windows and doors, and bouncy, sagging, uneven, and/or soft floors are the most common signs indicating that differential settlement has already occurred in your home.

But because only an expert can properly evaluate differential settlement and identify the root cause of the problem, our recommendation is to have your foundation inspected by one of our experienced professionals, before the symptoms become major. While nearly any type of foundation damage can be fixed, addressing foundation problems early is often the easiest and more affordable approach!